According to a first related art method, a shaft-shaped product is manufactured. Specifically, an engine valve is manufactured by hot forging. In this manufacturing method, an end portion of a blank obtained by cutting an elongated material is forged multiple times using a transfer press, whereby the end portion of the blank is enlarged step by step so as to be formed into a head portion having a relatively large diameter (see, e.g., JPH10-323735A).
According to a second related art method, a composite member is manufactured. Specifically, a stepped member is manufactured by joining a first member forming a head portion and a second member forming a shaft portion to each other. In this manufacturing method, an end portion of the second member is inserted into a bottomed hole provided in the first member, and shearing stress is provided repeatedly while providing compression stress in the second member in the axial direction, whereby the second member is enlarged inside the bottomed hole of the first member and the first member serving as the head portion is pressure welded to the end portion of the second member serving as the shaft portion (see, e.g., JP2013-99771A).
According to a third related art method, a shaft member of an outer joint for a constant speed universal joint is manufactured. Specifically, the shaft member to be welded to the bottom wall of a cup-shaped mouth portion is manufactured. In this manufacturing method, a bar workpiece is subjected to a shaft-diameter enlarging process to enlarge the intermediate portion of the bar workpiece, and the enlarged portion is cut and divided into two shaft members, whereby the divided enlarged portion remaining in each shaft member serves as a head portion having a relatively large diameter (see, e.g., JP2012-229714A).
The first related art method has excellent productivity because the head portion is provided by the forging process which is excellent in processing speed. However, since a metal mold is required for the forging process and the expense for the metal mold is high, cost is high in a case of a relatively small lot production.
The second related art method does not require a metal mold because the head portion is provided by the shaft-diameter enlarging process. However, the processing speed of the shaft-diameter enlarging process is generally slower than a forging process, so that there is a concern of low productivity.
The third related art method also does not require a metal mold because the head portion is formed by the shaft-diameter enlarging process and the cutting process. Further, in this method, since two shaft members are manufactured from a single bar workpiece, the disadvantage in processing speed due to the shaft-diameter enlarging process can be overcome.
However, in the third related art method, the bar workpiece is a solid workpiece. In this case, the enlarged portion that is enlarged by the shaft-diameter enlarging process and then cut by the cutting process is also solid, so the cutting process requires a long time, and there is a concern of low productivity due to the cutting process.